This application claims priority from Korean Patent Application No. 2002-71397, filed on Nov. 16, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a non-aqueous electrolytic solution and a lithium battery employing the same, and more particularly, to a non-aqueous electrolytic solution capable of stabilizing lithium metal and improving lithium ionic conductivity and a lithium battery employing the same.
2. Description of the Related Art
With the increasing need for smaller, lighter portable electronic devices, such as camcoders, portable communications devices, and portable computers, the need for more compact, lighter, higher capacitance batteries as a driving source of such portable electronic devices is increasing and considerable research into such batteries is being conducted.
Currently commercially available lithium ion secondary batteries utilize carbon as an anode active material and transition metal oxide, mostly LiCoC2, as a cathode active layer. Carbon, which is used as an anode active material, has a theoretical energy density of 372 mA/g, which is considerably lower than lithium metal that has an energy density of 3680 mA/g.
Lithium metal batteries utilize lithium metal, instead of carbon, as an anode active material. The volume and weight of batteries can be considerably reduced when lithium metal, instead of carbon, is utilized as an anode active material. This is the most important advantage of lithium metal batteries and is the main reason for a great deal of research into lithium metal batteries.
However, lithium metal batteries have problems of rapid capacitance drop with frequent charging/discharging cycles, volume change during charging/discharging, and safety concerns. These problems are caused by the dendric growth of lithium during charging/discharging. Lithium metal has a lowest density of 0.53 g/cm2, a greatest potential difference of −3.045V compared to a standard hydrogen electrode, a highest energy density of 3860 mAh/g, among other metals. Despite these advantages of lithium metal, secondary batteries with an anode made of lithium metal are not commercially available yet.
In order to solve the problem of dendric growth of lithium during charging/discharging, considerable research has been conducted. Accordingly, methods of stabilizing lithium by preventing lithium from growing into dendrites can be classified into two categories: physical methods, which involve forming a protective layer, and chemical methods.
Besenhard et al. have found that the structure of lithium greatly depends on the chemical composition and physical structure of a film deposited thereon and that physical and chemical non-uniformity of the film on the surface of the lithium electrode leads to the formation of lithium (J. of Electroanial. Chem., 1976, 68, 1).
Recently, Yoshio et al., have improved the reversibility of the lithium anode by controlling the surface state of the lithium anode(37th Battery Symposium in Japan). This research involves utilizing additives in an electrolytic solution or the lithium metal anode, so as to improve the surface properties of the lithium metal anode. As an example, the surface properties of the lithium metal anode have been effectively improved by forming on the lithium metal anode a dense, thin, even surface layer, which contains carbon dioxide, 2-methylfurane, magnesium iodide, benzene, pyridine, hydrofurane, or a surfactant. The purpose of this research was to prevent lithium dendrites from growing on the lithium metal anode through the formation of an even protective layer having higher lithium ionic conductivity to induce uniform current distribution over the lithium metal anode.
K. Naoi et al. have suggested the adsorption of polyethyleneglycol dimethylether onto the surface of the lithium anode as a protective coating, based on the fact that a core portion of a spiral ethylene oxide chain in polyethyleneglycol dimethylether can serve as a lithium ion path during charging/discharging (J. of Electrochem. Soc., 147, 813 (2000)). M. Ishikawa et al. have showed that the addition of aluminum iodide (AII3) or magnesium iodide (MgI2) in an organic electrolyte leads to the formation of a lithium alloy through reaction with the lithium anode, suppresses the growth of lithium dendrites, and improves the change/discharge efficiency (M. Ishikawa et al., J. of Electrochem., 473, 279(2000)). However, such protective layers are damaged during frequent charging/discharging operations or when they are soaked in an electrolytic solution for a long time.